The present invention relates generally to a vehicle brake control system and, in particular, to an anti-lock control system which, under anti-lock braking conditions, functions to maintain maximum braking force to at least one selected wheel of a vehicle by cyclically relieving and reapplying pressure to the selected wheel brake in order to maintain the wheel speed within a selected slip range.
Braking a vehicle in a controlled manner under adverse conditions such as rain, snow, or ice generally requires precise application of the brakes by the vehicle driver. Under these conditions, or in panic stop situations, a driver will often apply excessive brake pressure, thus causing one or more wheels to lock such that excessive slippage between the wheels and the road surface takes place. Such wheel lock-up conditions can lead to loss of directional stability and, possibly, uncontrolled vehicle spinout.
In a continuing effort to improve the operational safety of vehicles, many companies have been actively involved in the development of anti-lock braking systems. While typically such systems are adapted to control the braking of each braked wheel of the vehicle, some systems have been developed for controlling the braking of only a portion of the braked wheels.
Generally, prior art anti-lock systems include a central control unit for monitoring the speed of the controlled wheels to determine when an impending lock-up condition is present. When the brakes of the vehicle are applied and a predetermined slippage has been detected at the controlled wheels, the central control unit functions to control the application of hydraulic pressure to the associated brakes to prevent lock-up of the controlled wheels. Typically, the anti-lock system includes means for cyclically relieving and reapplying pressure to the associated brakes to limit wheel slippage to a safe level, while continuing to produce adequate brake torque to decelerate the vehicle as desired by the driver. Generally, the means for reapplying pressure is a separate hydraulic power source, and solenoid actuated valves are utilized to control the application of hydraulic pressure to the selected wheel brake.